SUMMARYCryostat sections of skin biopsies from five patients with chronic photosensitivity dermatitis with actinic reticuloid syndrome (PDAR) have been examined immunohistologically by the alkaline phosphatase: anti‐alkaline phosphatase staining technique using a panel of 24 monoclonal antibodies against lymphoid cells and their subsets. The lymphoid infiltrates in all cases had an essentially identical cellular composition, containing a mixture of T‐lymphocytes, T‐cell accessory cells (Langerhans cells) and other types of HLA‐DR positive dermal macrophages. In two patients there was an excess of T‐helper/inducer cells relative to T‐suppressor cells, while in the other three patients the numbers of T‐cells in these two subsets were approximately equal. Many of the infiltrating T‐cells expressed activation (HLA‐DR, interleukin‐2 receptor) or proliferation (the Ki67 nuclear antigen, transferrin receptor) associated markers. These data indicate that a T‐cell immune response is operative in c